Goodbye Cassini and WELL DONE!by John HlynialukIntentionally crashing a 5600 kg spacecraft into a planet does not sound like a good thing, but controllers at the NASA Jet Propulsion Lab in Pasadena, California will do just that to end a multi-billion (that “b” is not a typo) dollar mission that has been studying Saturn for the last 13 years. It happens on Sep 15 and it is, in fact, the smart thing to do. One of the discoveries made by Cassini, the vehicle in question (think fully-loaded, over-sized SUV), is that one of the moons of Saturn probably has an ocean under its ice layer that could harbour some form of life. If the spacecraft contaminated that moon with earthly bacteria (spacecraft are routinely sterilized but you can’t keep a hardy bug down), it would not be a good thing. It is much wiser to vapourize the vehicle intentionally in Saturn’s atmosphere where the incineration would reduce everything to sterile atoms.

But even to the very end Cassini’s instruments will be wringing out information about Saturn.

To quote Linda Spilker, Cassini project scientist from NASA’s Jet Propulsion Laboratory: The Cassini mission has been packed full of scientific firsts, and our unique planetary revelations will continue to the very end of the mission as Cassini becomes Saturn’s first planetary probe, sampling Saturn’s atmosphere up until the last second. We’ll be sending data in near real time as we rush headlong into the atmosphere – it’s truly a first-of-its-kind event at Saturn.

The data about the composition of Saturn’s atmosphere will be added to the wealth of other data and terabytes of images sent back by the spacecraft over the last 13 years. The discoveries have increased our knowledge about the planet immensely; where we had a single chapter in astronomy texts on Saturn, now there are literally dozens of volumes of information about the planet. As of December 2016, there were 3700 papers published in scientific journals using the data from this mission and it is not over just yet.

Cassini has clearly transformed our knowledge of the planet. Starting with the beautiful feature visible in telescopes from Earth, Saturn’s rings, Cassini found a highly dynamic system of particles constantly changing over time. Another surprise were the small moons embedded in the rings; these carve out gaps leaving behind beautiful sinuous patterns in their wakes. The dynamics of the rings of Saturn have revealed secrets about how planets form around stars and give insights into how our own planet may have coalesced from the dust circling our Sun in our early solar system.

As some discoveries have solved mysteries about Saturn, other mysteries have arisen as scientists scramble to analyze the data coming in. This includes giant hurricanes at Saturn’s poles, one with bizarre hexagonal sides unlike anything ever seen. How can this pattern be maintained over time? The number of scientific papers will continue to grow as planetary meteorologists propose theories to explain this unusual structure and other weather patterns in Saturn’s immense atmosphere.

Cassini also studied the dozens of moons circling Saturn and discoveries of wonderful things have involved them as well. Plumes of water vapour stream up from Saturn’s moon Enceladus, indicating a sub-surface ocean that is a possible abode for living organisms perhaps like those near Earth’s own deep ocean vents, the “black smokers”. The Cassini mission to Saturn also involved a smaller spacecraft called Huygens, which piggy-backed on Cassini from Earth and was released to parachute into the atmosphere of Titan, Saturn’s most enigmatic moon. There it found hydrocarbon lakes and rivers containing organic compounds, -a world where the chemistry may resemble our early Earth giving us a possible look back at our own evolution. I highly recommend a look at the NASA Cassini website for more about the spacecraft and its discoveries, including some of the most spectacular images of Saturn, its rings and moons that I have ever seen. The link is provided here: https://saturn.jpl.nasa.gov